feat: remove legacy native AI and vector stack for rc.7

[Proxicon]

Summary

• Remove RatelDesk's native in-process AI provider, vector retrieval, knowledge-base, suggestion, reply-draft, and AI change-review surfaces.
• Preserve the external Netclaw AiAssistant SignalR ticket chat, webhook worklogs, and MCP HTTP/stdio transports.
• Move bundled and test PostgreSQL to standard postgres:16; fresh and upgraded databases no longer require pgvector or pg_trgm.
• Add forward PostgreSQL and SQLite cleanup migrations. They remove the retired schema without dropping PostgreSQL extensions.
• Bump the product prerelease suffix to 0.1.0-rc.7 and update setup/self-hosting documentation.

Validation

• dotnet build tests/Helpdesk.Tests/Helpdesk.Tests.csproj -c Release --no-restore
• PostgreSQL migration regression (ChatMigrationTests) against postgres:16
• SQLite migration regression (SqliteProviderRegressionTests)
• External SignalR chat persistence tests (ChatPostgresTests) against postgres:16
• MCP catalog/HTTP transport tests (McpOperationCatalogTests, McpHttpHostTests)
• Dependency/configuration audit confirms no native AI/vector package or extension requirement remains.

Disclosure check

• No credentials, customer data, employee data, private URLs, internal hostnames, or deployment details are included.